Spinal Cord Stimulation

What is Spinal Cord Stimulation?

Spinal Cord Stimulation (SCS) is a minimally invasive procedure that involves implanting a device that applies low currents of electrical stimulation to the spinal cord and/or its exiting nerves.

Spinal cord stimulation is referred to by some pain experts as a “pacemaker for pain”. It works by sending small electrical impulses created by a compact generator through thin leads, or electrical cables, to the spinal cord, where they block pain signals traveling to the brain. Pain is replaced with a mild tingling or a massaging sensation, called paresthesias. A wireless remote control is used to adjust the location and degree of stimulation by selecting pre-programmed settings.

The core technology that is used in today’s SCS systems was developed in the mid-1960s. Melzack and Wall developed the original theory for the mechanism of spinal cord stimulation in 1965 (1). This “gate-control theory” for pain proposes that simultaneously triggered touch and vibratory sensation inhibits pain stimuli sensation due to their shared location in the spinal cord, the dorsal horn nucleus. In essence this theory is the foundation for spinal cord stimulation. An everyday example of this theory is seen when one has a headache. Many people will rub their temples or another area of their head, stimulating the muscles of the head or sensory fibers of the skin. When these areas are stimulated, to some degree they block the sharp pain perceived from an active headache. This is also commonly seen when you accidently bump your knee, elbow, or finger and you rub the associated area inhibiting the acute painful stimuli to the brain.

The first spinal cord stimulators were implanted directly on the dorsal column of the spinal cord of terminal cancer patients by Shealy et al. (2) in 1967. Shortly after, Shimogi et al. were the first to publish the successful implementation of epidural spinal cord stimulation (3), which is a percutaneous, less invasive technique. This avoids the complications of the original open surgery, which includes cerebrospinal fluid leakage, localized fibrosis, and arachnoiditis. Another initial challenge was a limited area covered by the single, or monopolar electrode. SCS leads today have evolved from monopolar (1 active electrode) to bipolar (2 active eletrodes), quadripolar (4 active electrodes), and octapolar (8 active elctrodes) leads.

Anatomy

The spinal cord is a bundle of nervous tissue and supporting material that extends from the brain to innervate the rest of the body. The brain and the spinal cord together form the central nervous system (CNS), which sends and receives messages from the body through the peripheral nervous system (PNS).

The spinal cord is contained in the spinal canal formed by the vertebral column. The meninges are a covering consisting of three layers that continues from the cranium to the sacrum and protects the spinal cord and its nerves. The innermost layer, or pia mater, wraps around the brain and spinal cord. The middle layer, or arachnoid mater, is a spider web-like layer. The outermost layer is called the tough dura mater. Between the arachnoid mater and the pia mater is ones cerebral spinal fluid (CSF) which protects and buffers the brain and spinal cord. Outside of the three-layered meninges is the epidural space. The epidural space is a potential space that lies outside of the dura and typically houses protective fatty tissue and blood vessels. The epidural space is where medications are placed for epidural blocks and where the leads are placed for spinal cord stimulation.

The spinal cord normally extends from the foramen magnum, a hole at the bottom of the cranium, to the L1 vertebra in adults. In children the spinal cord ends at L3 and travels upward as they grow older. Nerve roots exit from the back and front of the spinal cord and then join to form the spinal nerves from second cervical nerve root to the fifth sacral nerve root (C2 to S5). The spinal nerves then leave the spinal canal through openings between each vertebra called the intervertebral foramen.

The main functions of the spinal cord are to relay signals from the brain to all muscles for movement, relay signals up from the body for sensory input, and to also coordinate reflexes. Each spinal nerve root supplies sensory innervations to a specific area of skin called a dermatome.

Lesions of specific nerve roots result in predictable pattern of signs and symptoms. For instance, a lesion in a lumbar nerve root may cause radiating pain, muscle weakness, numbness, tingling, and/or reflex changes in the legs. This gives physicians a tool for localizing the lesion that is causing the symptoms. Imaging modalities such as MRIs and diagnostic tools like EMGs (electromyography) may be used to assist and confirm a diagnosis.

Description

SCS is a minimally invasive procedure that is done on an outpatient or short hospital stay basis. There are two steps to the procedure, a trial procedure and a permanent implant. The trial procedure is a brief procedure that is usually done under light sedation. The area where the leads are implanted is numbed with local anesthesia. One or more needles are directed under x-ray guidance into the epidural space, which is the area surrounding the spinal cord and/or nerve roots.

The leads are then placed through the needles and are steered to the desired location. The leads are then connected to an external generator. Once the generator is turned on, the impulses are sent with varying intensities to different positions on the leads. The patient will sense paresthesias (tingling and/or buzzing) while a programmer adjusts the settings to provide the “optimal coverage” over the painful areas. These settings are then saved on the remote control so that the patient can adjust the stimulator to best fit their needs. The trial period usually lasts a few days to a week, depending on the effectiveness of the SCS. The goal is to test the stimulator’s effectiveness at relieving pain, increasing normal daily activities, and overall improving quality of life.

If SCS trial period provides sufficient pain relief (usually 50% or greater), then the trial is deemed a success and a permanent implant may be scheduled if the patient desires. The permanent procedure is carried out in similar fashion to the trial procedure. The significant difference is that the generator is implanted, like a pacemaker, in a subcutaneous pocket. Usually the upper, outer quadrant of the buttock is chosen, which results in a largely unrecognizable and comfortable experience. A different type of lead may also be used if your physician believes this will give you better long-term relief. A few small bandages will be applied over the incisions. After the procedure, the patient can usually return home the same or the following day.